^{1}

^{1}

The problems of formation of neutron fields are using the radionuclide neutron sources. Theoretical estimates of the contribution to the values of the neutron flux density, kerma and ambient dose equivalent scattered neutron radiation under various conditions of use of neutron sources. The coefficients for calculating the values of different variables at different distances from the source are using the measured values at certain distances from the source. The values of the average energies for neutron spectra are obtained using the radionuclide neutron sources.

The most significant influencing factor of calibration of neutron radiation measuring instruments is scattered radiation which appears in the room where the calibration rig with a neutron source is located (from the walls, floor, ceiling and various items). It leads to a change of the neutron radiation spectrum with a distance from the source of neutron radiation on account of different contributions of the scattered radiation, which leads to the violation of the inverse square law. This fact is true for both an open geometry and a collimated geometry [

In general, for a collimated geometry the value of _{sci}―from a distributed source of radiation scattered in the room), i.e.:

To transfer measurement units (flux density (FD), kerma, ADE) for the certification of installations it is necessary to carry out measurements for the entire range of working distances of the setup [

Considering the presence of objects which dissipate and reduce radiation, Equation (1) can be rewritten in the form:

where

It is obvious that the values of

The calculated values of

Calculations of the values of

The calculated values of

To calculate

The neutron source | Density of flux with energy: | Total flux density | Kerma | ADE | ||
---|---|---|---|---|---|---|

less than 0.5 МэВ | more than 0.5 МэВ | more than 1 МэВ | ||||

PuBe | 0.609 | 0.208 | 0.195 | 0.284 | 0.189 | 0.220 |

The results of calculations of

Amendments for distances which are given in GOST 8.355-79 are not satisfactory for calculating the values according to the inverse square law [

During the certification of installations it is advisable to carry out measurements at two different distances from the center of neutron radiation: of 100 cm and 150 cm. The measurement result obtained at a distance of 100 cm is used to calculate the values at different distances by using the transition coefficients which are shown in the respective charts. The measurement result corresponding to the distance of 150 cm is used to verify that the position of the center of the source matches the zero point of the calibration scale of the installation. The following condition should be satisfied:

where

F is the transition coefficient of the corresponding value from the point at the distance from the source of 100 cm to the point at the distance from the source of 150 cm;

∆ is a percent error of the measuring instrument used for certification;

δ is an expanded uncertainty of calculation of coefficient F with a coverage rate of 2 (less than 1%).

The value of the

When the measuring instruments are tested, especially when an energy dependence of the sensitivity of measuring instruments is determined, the important factor is a possibility of modifying the spectra of neutron radiation. In

Averaging in magnitude: | Average energy for the types of spectra generated with the PuBe source: | |||||
---|---|---|---|---|---|---|

1 | 2 | 3 | 4 | 5 | 6 | |

The density of flux of neutrons with energy more than 0.414 eV | 3.41 | 2.23 | 0.96 | 3.33 | 1.05 | 1.33 |

Power of ADE | 3.90 | 3.22 | 1.83 | 3.86 | 1.91 | 3.26 |

Power of kerma | 4.50 | 4.00 | 2.30 | 4.48 | 2.33 | 3.95 |

Power of effective dose | 4.31 | 3.77 | 2.19 | 4.30 | 2.26 | 3.68 |

Averaging in size: | Average energy for the types of spectra generated with the Cf-252 source: | |||||
---|---|---|---|---|---|---|

1 | 2 | 3 | 4 | 5 | 6 | |

The density of flux of neutrons with energy more than 0.414 eV | 1.89 | 0.90 | 0.50 | 1.75 | 0.60 | 0.53 |

Power of ADE | 2.26 | 1.68 | 1.15 | 2.17 | 1.23 | 2.03 |

Power of kerma | 2.71 | 2.08 | 1.36 | 2.63 | 1.46 | 2.51 |

Power of effective dose | 2.62 | 2.00 | 1.33 | 2.54 | 1.44 | 2.04 |

In

1) the source is placed in the container of the UCPN-1M installation with the collimator;

2) the source is placed in the container of the UCPN-1M installation with a polyethylene moderator;

3) the source is placed in the container of the UCPN-1M installation with the moderator of the form of a cone of iron of 19 cm long;

4) the source is placed in an open geometry;

5) between the source and the point under consideration there is placed an absorbent cone made of polyethylene with 5% (by weight) of natural boron of 50 cm long;

6) the source is placed in the center of the sphere of a 30 cm diameter filled with heavy water.

These spectra were obtained at a distance of 1 m from the centre of the source in the room of a size

S. G. Fedorov,P. F. Maslyaev, (2015) Formation of Neutron Fields from Radionuclide Neutron Sources for Metrological Support of Dosimetric Quantities Measurements. Journal of Applied Mathematics and Physics,03,937-941. doi: 10.4236/jamp.2015.38114